The term "thin film" as used herein in the description and the claims means conductive layers or coatings applied by vacuum deposition processes, such as sputtering deposition, that are very thin, for example on the order of about only 100 Angstroms to 5,000 Angstroms, or about 10 to 500 nanometers, thick. A vacuum deposition manufacturing process is capable of producing thin film layers that are very uniform, electrically conductive and which adhere well to plastic film substrates. Thin film conductive layers applied to flexible plastic film substrates can be usefully employed in electronic applications, such as membrane switches, keyboard switchcores and other flexible circuit products in which properties such as light weight, flexibility and surface mount capability are useful features. Electrical elements incorporating conductive thin film layers can be powered by AC and DC power sources, as appropriate to the specific installation under consideration.
A flexible transparent thin film electrical heater panel is a product that is starting to become of interest to manufacturers of various electronic components that require heat at low to moderate temperatures in order to function properly or in which a flexible heater panel is needed that has a special shape or must fit on a flat or curved surface. A heater panel of this general type presently commercially available comprises a gold thin film layer vacuum deposited onto a plastic film substrate. However, a gold thin film flexible heater panel has a number of disadvantageous characteristics that limit its utility. For example, a gold thin film conductive layer does not have a high degree of visible light transmission. This makes it necessary to apply a very thin layer such as only about 100 Angstroms thick on the substrate in order to have even a minimum degree of visible light transmission, which at best is only on the order of about 40% or so. Thus, gold thin film flexible heaters do not have the high degree of light transmission sought for many uses of this type of product. Further, the gold thin film coating imparts a yellowish color or cast to the transmitted light, and it cannot transmit light without changing its color. Also, a gold thin film conductive layer, particularly of the foregoing thickness, is mechanically fragile. This presents a problem because a flexible plastic film heater is often affixed to a surface of an object by means of transparent pressure sensitive adhesive that is coated over the thin film conductive layer. A gold thin film conductive layer, however, is sufficiently fragile that it often cracks and breaks down structurally when subjected to stress of this type of affixation, and thereby rendered useless for its intended application. Another disadvantage that is of great significance for this type of product is that a gold thin film has a very limited power range. The low resistance of gold means that prior art heaters of this type have a very low power density range, so that the maximum power density of gold thin film is only about one to two watts per square inch. Other adverse factors include questionable environmental durability and relatively high expense due to the cost of gold.